HDU 1394-Minimum Inversion Number(BIT)

#include <map>
#include <set>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <vector>
#include <string>
#include <cctype>
#include <complex>
#include <cassert>
#include <utility>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <algorithm>
using namespace std;
typedef pair<int,int> PII;
typedef long long ll;
#define lson l,m,rt<<1
#define pi acos(-1.0)
#define rson m+1,r,rt<<11
#define All 1,N,1
#define read freopen("in.txt", "r", stdin)
#define N 5010
const ll  INFll = 0x3f3f3f3f3f3f3f3fLL;
const int INF= 0x7ffffff;
const int mod =  1000000007;
int bit[N],a[N],n;
int sum(int x){
    int num=0;
    while(x>0){
        num+=bit[x];
        x-=(x&(-x));
    }
    return num;
}
void add(int x,int d){
    while(x<=n){
        bit[x]+=d;
        x+=(x&(-x));
    }
}
int main()
{
    while(~scanf("%d",&n)){
        for(int i=0;i<n;++i){
            scanf("%d",&a[i]);
        }
        memset(bit,0,sizeof(bit));
        int total=0;
        for(int i=n-1;i>=0;i--){
            total+=sum(a[i]+1);
            add(a[i]+1,1);
        }
        int minv=total;
        for(int i=0;i<n;++i){
            total+=n-1-2*a[i];
            minv=min(minv,total);
        }
        printf("%d\n",minv);
    }
return 0;
}